Chronic back problems can cause pain and disability for a large segment of the population. Frequently, the cause of back pain is traceable to diseased disc material between opposing vertebrae. When the disc material is diseased, the opposing vertebrae may be inadequately supported, resulting in persistent pain.
Surgical techniques have been developed to remove the diseased disc material and fuse the joint between opposing vertebral bodies. Arthrodesis or fusion of the intervertebral joint can reduce the pain associated with movement of an intervertebral joint having diseased disc material. Generally, fusion techniques involve removal of the diseased disc, distracting the intervertebral joint space, drilling a bore for receiving the implant and inserting the implant between the opposing vertebral bodies.
Spinal fusion implants and related surgical instruments for implanting a fusion device are known and disclosed in, for example, U.S. Pat. Nos. 5,741,253; 5,658,337; 5,609,636; 5,505,732; 5,489,308; 5,489,307; 5,484,437; 5,458,638; 5,055,104; 5,026,373; 5,015,247; 4,961,740 and 4,501,269. The disclosure of each of these patents are incorporated herein by reference. Procedures for fusing an intervertebral joint space typically include placement of at least two cylindrical implants in parallel arrangement between the opposing vertebrae.
Some presently available systems for intervertebral fusion provide for preparing an implant site through a hollow tube. Procedures for preparing an implant site through a hollow tube are shown in, for example, U.S. Pat. Nos. 5,484,437; 5,489,307 and 5,505,732. The disclosure of each of these patents are incorporated herein by reference. In some procedures, the implants are also inserted into the prepared site through the hollow tube. Preparing the implant site by passing instruments through a hollow tube advantageously provides for an isolated surgical field with reduced chance of injury to soft tissues surrounding the surgical site.
Often times, the degenerative changes of the diseased disc cause a collapse of the intervertebral disc space. Thus, prior to implantation, the intervertebral disc space may be distracted to restore the normal height of the disc space or the normal relationship between the vertebrae to be fused. Maintaining the restored disc space height and/or vertebral relationships throughout preparation of the implant site is important for the ultimate stability at the fusion site. This may be particularly true when a lordotic implant is used to restore or establish a particular degree of lordosis between adjacent vertebrae.
However, most presently available implant procedures do not take into account events which can cause deviation of the final implant position from the position selected by the surgeon during initial implant site determination. Deviation of the final implant position from the initially selected position can result in, for example, reduced fusion site stability, implant migration, implant loosening, implant subsidence and improper purchase of an anchoring arrangement of an implant (e.g., threads, spikes, ridges, knurls, etc.), at the fusion site. In the case of a lordotic implant, deviation of the final implant position can also cause deviation from the desired degree of lordosis, deviation in the amount of distraction (i.e., disc space height), and/or deviation in the amount of purchase of the implant anchoring arrangement into the surrounding cortical bone.
Thus, while present procedures for implantation through a hollow tube help to reduce the chance of iatrogenic tissue trauma caused by the implant procedure, the overall results of the implant may be not be optimal due to a deviation of the final placement of the implant from the desired placement position.
Accordingly, there is a need for instrumentation and methods which insure that the final position of a spinal implant is in the position which was initially established by the surgeon during distraction of the intervertebral joint space. The present invention is directed to this need.